1. Field of the Invention
This invention relates to an output power shaft structure for a personal watercraft.
2. Description of Background Art
Conventionally, the power source of a widespread personal watercraft is a 2-cycle engine. However, in order to achieve a reduction of public nuisance and reduction of noise in recent years, a personal watercraft which uses a 4-cycle engine as its power source has been proposed (Japanese Patent No. 2880691).
In the personal watercraft mentioned above, as shown in FIG. 11 of the present invention, an engine 1 is carried on a watercraft body 2 such that a crankshaft 1a thereof is directed in a longitudinal direction of the watercraft body 2. A shaft 3a of a propulsion device 3 (for example, a jet pump) is coupled to a rear end of the crankshaft 1a on an extension line of the crankshaft 1a by means of a coupling 4. Accordingly, power of the crankshaft of the engine 1 is transmitted to the propulsion device 3 through the shaft 3a of the propulsion device 3. The watercraft body 2 is propelled by the propulsion device 3.
In the personal watercraft having such a configuration as described above, a reactive force from the propulsion device 3 is transmitted to the crankshaft 1a of the engine 1. In particular, the crankshaft 1a is acted upon, via the coupling 4 through the shaft 3a of the propulsion device 3, by a countertorque, thrust force, bending force (force tending to deflect the crankshaft) and vibrations. Such forces as just mentioned act in a complicated combination upon the crankshaft 1a. 
The conventional personal watercraft described above is structured such that a rear end 1b of the crankshaft 1a is projected to the outside of the engine case and the shaft 3a of the propulsion device 3 is connected to the rear end 1b of the crankshaft 1a through the coupling 4. Accordingly, the engine 1 has a length increased by a length over which the rear end 1b of the crankshaft 1a is projected to the outside of the engine case.
Therefore, when such a countertorque, thrust force, bending force and vibrations as mentioned above act in a complicated combination upon the crankshaft 1a, in the background art, the crankshaft 1a is likely to suffer from flexure (deflection), since the crankshaft 1a is longer. Since the crankshaft 1a rotates at a high speed, fatigue is likely to occur with the crankshaft 1a and a bearing portion of the crankshaft 1a. As a result, the durability of the engine can be degraded.
The object of the present invention resides in a solution to such a problem as described above, wherein an output power shaft structure for a personal watercraft is superior in durability.
In order to attain the object described above, according to a first aspect of the present invention, an output power shaft structure for a personal watercraft includes an engine carried on a watercraft body such that a crankshaft thereof is directed in a longitudinal direction of the watercraft body. A shaft of the propulsion device is connected to a rear end of the crankshaft of the engine on an extension line of the crankshaft. The shaft includes the rear end of the crankshaft disposed in an engine case. An output power shaft is separate from the crankshaft and is connected to the rear end of the crankshaft. The shaft of the propulsion device is connected to a rear end of the output power shaft.
According to the first aspect of the present invention, the rear end of the crankshaft is disposed in the engine case. Therefore, the crankshaft can be formed shorter that of the background art. Accordingly, even if a countertorque, thrust force, bending force and vibrations (as mentioned above) act in a complicated combination upon the crankshaft, the crankshaft is less likely to suffer from flexure (deflection), since the crankshaft is shorter. As a result, fatigue is less likely to occur with the crankshaft, which rotates at a high speed, and a bearing portion of the crankshaft. Accordingly, the durability of the engine is augmented.
According to a second aspect of the present invention, the engine is a 4-cycle OHC engine. A transmission chamber in which a transmission mechanism for transmitting power of the crankshaft to a camshaft of the engine is provided at a rear portion of the crankshaft of the 4-cycle OHC engine. The rear end of the crankshaft and a front end of the output power shaft are connected to each other in the transmission chamber. Accordingly, the rear end of the crankshaft and the front end of the output power shaft can be connected to each other making the most of the space in the transmission chamber.
According to a third aspect of the present invention, a driving sprocket wheel or a driving gear of the transmission mechanism is formed integrally on the output power shaft separate from the crankshaft. Accordinlgy, the driving sprocket or the driving gear of the transmission mechanism can be readily provided when compared with the driving sprocket or driving gear is provided on the crankshaft.
According to a fourth aspect of the present invention, a pulser rotor is formed integrally with the output power shaft separate from the crankshaft. Accordinlgy, the pulser rotor can be readily provided when compared with another case where the pulser rotor is provided on the crankshaft. In addition, a pulse can be extracted making the most of the space in the transmission chamber.
According to a fifth aspect of the present invention, a spline is formed on the rear end of the crankshaft and a front end of the output power shaft. The rear end of the crankshaft and the front end of the output power shaft are connected to each other through a connecting pipe having a spline for engaging with the splines formed on an inner face thereof. Accordingly, power from the crankshaft is transmitted with certainty to the output power shaft through the connecting pipe.
Although a countertorque, a bending force and vibrations from the shaft of the propulsion device are transmitted to the crankshaft through the connecting pipe, since the connection between the three members is established by the spline fitting, a reactive force from the shaft of the propulsion device, particularly a bending force and vibrations, are reduced significantly in the process wherein they are transmitted from the output power shaft to the connecting pipe and further to the crankshaft. Accordingly, with the output power shaft structure for a personal watercraft according to the fifth aspect of the present invention, the power from the crankshaft is transmitted with certainty to the output power shaft through the connecting pipe. Furthermore, the durability of the engine is further augmented.
According to a sixth aspect of the present invention, a gap is formed between opposing faces of the rear end of the crankshaft and the front end of the output power shaft. Accordingly, a countertorque, a thrust force, a bending force and vibrations transmitted from the shaft of the propulsion device to the crankshaft, particularly the thrust force and the bending force, are further reduced. As a result, the durability of the engine is further augmented with a higher degree of certainty.
According to a seventh aspect of the present invention, a driving sprocket wheel or a driving gear of the transmission mechanism is formed integrally on the connecting pipe. Accordingly, the driving sprocket wheel or the driving gear of the transmission mechanism can be provided readily when compared with another case where driving sprocket wheel or the driving gear is provided on the crankshaft.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.